But here’s the executive summary, for you executives too busy to read the whole thing. It’s clearly written, short, and earth-shakingly important. I’ve put a few key passages in boldface.

EXECUTIVE SUMMARY

Emissions of carbon dioxide from the burning of fossil fuels have ushered in a new epoch where human activities will largely determine the evolution of Earth’s climate. Because carbon dioxide in the atmosphere is long lived, it can effectively lock the Earth and future generations into a range of impacts, some of which could become very severe. Therefore, emissions reductions choices made today matter in determining impacts experienced not just over the next few decades, but in the coming centuries and millennia. Policy choices can be informed by recent advances in climate science that quantify the relationships between increases in carbon dioxide and global warming, related climate changes, and resulting impacts, such as changes in streamflow, wildfires, crop productivity, extreme hot summers, and sea level rise.

Since the beginning of the industrial revolution, concentrations of greenhouse gases from human activities have risen substantially. Evidence now shows that the increases in these gases very likely (>90 percent chance) account for most of the Earth’s warming over the past 50 years. Carbon dioxide is the greenhouse gas produced in the largest quantities, accounting for more than half of the current impact on Earth’s climate. Its atmospheric concentration has risen about 35% since 1750 and is now at about 390 parts per million by volume, the highest level in at least 800,000 years. Depending on emissions rates, carbon dioxide concentrations could double or nearly triple from today’s level by the end of the century, greatly amplifying future human impacts on climate.

Society is beginning to make important choices regarding future greenhouse gas emissions. One way to inform these choices is to consider the projected climate changes and impacts that would occur if greenhouse gases in the atmosphere were stabilized at a particular concentration level. The information needed to understand such targets is multifaceted: how do emissions affect global atmospheric concentrations and in turn global warming and its impacts?

This report quantifies, insofar as possible, the outcomes of different stabilization targets for greenhouse gas concentrations using analyses and information drawn from the scientific literature. It does not recommend or justify any particular stabilization target. It does provide important scientific insights about the relationships among emissions, greenhouse gas concentrations, temperatures, and impacts.

CLIMATE CHANGE DUE TO CARBON DIOXIDE WILL PERSIST MANY CENTURIES

Carbon dioxide flows into and out of the ocean and biosphere in the natural breathing of the planet, but the uptake of added human emissions depends on the net change between flows, occurring over decades to millennia. This means that climate changes caused by carbon dioxide are expected to persist for many centuries even if emissions were to be halted at any point in time.

Such extreme persistence is unique to carbon dioxide among major agents that warm the planet. Choices regarding emissions of other warming agents, such as methane, black carbon on ice/snow, and aerosols, can affect global warming over coming decades but have little effect on longer-term warming of the Earth over centuries and millennia. Thus, long-term effects are primarily controlled by carbon dioxide.

The report concludes that the world is entering a new geologic epoch, sometimes called the Anthropocene, in which human activities will largely control the evolution of Earth’s environment. Carbon emissions during this century will essentially determine the magnitude of eventual impacts and whether the Anthropocene is a short-term, relatively minor change from the current climate or an extreme deviation that lasts thousands of years. The higher the total, or cumulative, carbon dioxide emitted and the resulting atmospheric concentration, the higher the peak warming that will be experienced and the longer the duration of that warming. Duration is critical; longer warming periods allow more time for key, but slow, components of the Earth system to act as amplifiers of impacts, for example, warming of the deep ocean that releases carbon stored in deep-sea sediments. Warming sustained over thousands of years could lead to even bigger impacts.

IMPACTS CAN BE LINKED TO GLOBAL MEAN TEMPERATURES

To date, climate stabilization goals have been most often discussed in terms of stabilizing atmospheric concentrations of carbon dioxide (e.g., 350 ppmv, 450 ppmv, etc.). This report concludes that, for a variety of conceptual and practical reasons, it is more effective to assess climate stabilization goals by using global mean temperature change as the primary metric. Global temperature change can in turn be linked both to concentrations of atmospheric carbon dioxide and to accumulated carbon emissions.

An important reason for using warming as a reference is that scientific research suggests many key impacts can be quantified for given temperature increases. This is done by scaling local to global warming and by “coupled linkages” that show how other climate changes, such as alterations in the water cycle, scale with temperature. There is now increased confidence in how global warming levels of 1°C, 2°C, 3°C etc. would relate to certain future impacts. This report lists some of these effects per degree (°C) of global warming, including:

• 5-10 percent changes in precipitation in a number of regions

• 3-10 percent increases in heavy rainfall

• 5-15 percent yield reductions of a number of crops

• 5-10 percent changes in streamflow in many river basins worldwide

• About 15 percent and 25 percent decreases in the extent of annually averaged and September Arctic sea ice, respectively

For warming of 2°C to 3°C, summers that are among the warmest recorded or the warmest experienced in people’s lifetimes, would become frequent. For warming levels of 1°C to 2°C, the area burned by wildfire in parts of western North America is expected to increase by 2 to 4 times for each degree (°C) of global warming. Many other important impacts of climate change are difficult to quantify for a given change in global average temperature, in part because temperature is not the only driver of change for some impacts; multiple environmental and other human factors come into play. It is clear from scientific studies, however, that a number of projected impacts scale approximately with temperature. Examples include shifts in the range and abundance of some terrestrial and marine species, increased risk of heat-related human health impacts, and loss of infrastructure in the coastal regions and the Arctic.

STABILIZATION REQUIRES DEEP EMISSIONS REDUCTIONS

The report demonstrates that stabilizing atmospheric carbon dioxide concentrations will require deep reductions in the amount of carbon dioxide emitted. Because human carbon dioxide emissions exceed removal rates through natural carbon “sinks,” keeping emission rates the same will not lead to stabilization of carbon dioxide. Emissions reductions larger than about 80 percent, relative to whatever peak global emissions rate may be reached, are required to approximately stabilize carbon concentrations for a century or so at any chosen target level.

But stabilizing atmospheric concentrations does not mean that temperatures will stabilize immediately. Because of time-lags inherent in the Earth’s climate, warming that occurs in response to a given increase in the concentration of carbon dioxide (“transient climate change”) reflects only about half the eventual total warming (“equilibrium climate change”) that would occur for stabilization at the same concentration. For example, if concentrations reached 550 ppmv, transient warming would be about 1.6°C, but holding concentrations at 550 ppmv would mean that warming would continue over the next several centuries, reaching a best estimate of an equilibrium warming of about 3°C. Estimates of warming are based on models that incorporate ‘climate sensitivities’—the amount of warming expected at different atmospheric concentrations of carbon dioxide. Because there are many factors that shape climate, uncertainty in the climate sensitivity is large; the possibility of greater warming, implying additional risk, cannot be ruled out, and smaller warmings are also possible. In the example given above, choosing a concentration target of 550 ppmv could produce a likely global warming at equilibrium as low as 2.1°C, but warming could be as high as 4.3°C, increasing the severity of impacts. Thus, choices about stabilization targets will depend upon value judgments regarding the degree of acceptable risk.

CONCLUSION

This report provides a scientific evaluation of the implications of various climate stabilization targets. The report concludes that certain levels of warming associated with carbon dioxide emissions could lock the Earth and many future generations of humans into very large impacts; similarly, some targets could avoid such changes. It makes clear the importance of 21st century choices regarding long-term climate stabilization.

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17 Responses to Climate Stabilization Targets

I have to say I haven’t read the NAS report, but I do like that the executive summary shows the importance of climate sensitivity.

I will use the opportunity to raise a question that I think is important. Rather than point estimates of climate sensitivity most reports are for probability distributions (or more crudely of estimates with error bars). Economists especially seem to be paying attention to the idea that we need to worry not about the median estimate (best guess) but the highest value at 1% (or sometimes less) chance. For example, commonly you hear it said that we buy insurance for much smaller risks than 1%.

For various reasons I think that, while taking account of the possibility of high climate sensitivity, for planning purposes the median value of 3 degrees C should be given most weight. I have various ethical and economic reasons for this position. But I also have a statistical concern – the tail of the probability distribution is poorly defined while the chances of further research narrowing the peak seem to me promising.

In one sense, the world needs lots more energy, the cleaner the better, as soon as possible, it doesn’t make much difference. But in the longer term, for big picture strategy it makes a surprisingly big difference. I think the economists are making something of a hash of this debate, and can provide references if people are interested. But I would be interested to see what mathematicians and physicists here make out of this debate on the science side:

For various reasons I think that, while taking account of the possibility of high climate sensitivity, for planning purposes the median value of 3 degrees C should be given most weight. But in the longer term, for big picture strategy it makes a surprisingly big difference. I think the economists are making something of a hash of this debate, and can provide references if people are interested.

I really appreciate your well-thought-out contributions. I definitely want to read those references you listed. But I’m afraid that it’s going to take me quite a while to catch up with everything you’ve pointed us to already!

I guess you’re the co-author of Energise!: A Future for Energy Innovation, right? I’m doing a series of email interviews for the new series of This Week’s Finds. Do you think you’d be interested? This might give me a way to ask questions about what you’ve said, for you to answer, and for the whole thing to be presented in a readable format — more readable than spread out over a bunch of blog comments, anyway.

But yes, I’m interested as to why you think economists have made a bit of a hash of this debate. I can see that in a world rationally planning for its future, there would be a big difference between basing our decisions on median values regardless of what happens at the 1% tail and trying to “insure” ourselves against things that have a 1% chance of happening. I don’t get the feeling that we live in such a world.

I would love to think more about such things, and also the issue of “tipping points”, which I’ve seen a lot of debate about.

I’m happy to help out if I can, although I would hesitate to describe myself as an expert. I’m in the final stretch of a PhD unrelated to climate / energy right now, so contributions will be a bit sporadic. Sort of ‘change is as good as break’ for my brain.

I co-wrote Energise!, which is more political than scientific, although of course we tried to make it scientifically informed and technically correct. Of things that have come up on this blog, I would say the book is closest to Freeman Dyson’s approach and specifically his remarks on humanism. I think a lot have people have labelled him a ‘denier’ of climate science when they are actually reacting to some of his ethical and political positions.

What is the real distance between Freeman Dyson suggesting genetically modified trees that grow diamonds and environmentalists suggesting that we learn from traditional Amazonian agriculture how to sequester carbon as biochar? In large part I think the difference is in attitudes toward technology and tradition.

These are not unimportant when it comes to deciding what should be done. But knowing that politics is important doesn’t mean that you can’t also step back and work out what science can and can’t tell us about climate and energy. From what I’ve seen so far it’s looking optimistic that Azimuth could help clarify things, which in itself would be no small contribution!

Diamond growing trees… Until we get these we need to to use conventional trees and stone age tech (making charcoal), plus reduce emissions. The bio technologists can’t even make photosynthesis more efficient. (I bet even that “simple” task would take as long as getting a thermonuclear reactor running – way too long. And by Liebig’s law perhaps it’s unrealistic, anyhow.)

Here’s my gut feeling, which I intend to study in a critical way. It’s possible that futuristic technologies (genetic engineering far beyond what we can do now, nanotechnology, and the like) won’t come online fast enough to stop CO2 levels from reaching dangerously high values, but that later they might be used to reduce the CO2 levels from the high values they have reached, assuming that our civilization hasn’t gone into a catabolic collapse of a sort that prevents these technologies from being developed and deployed.

The short answer is that the error bars around any of the climate model projections are large enough to make them worthless. The base case scenario of zero feedback CO2 warming causes no problems for the globe. We’ve already had a near doubling of CO2 and the planet is just as hospitable as ever. Another doubling won’t put us out of business. Organizations like the IPCC and NAS feel compelled to fudge this and present a “consensus”.

There may be a lot of good reasons to replace oil and hydrocarbons (e.g. national and economic security) – computer projections of models 100 yrs from now are not one of them.

Understanding collapse possibilities is important. Understanding collapsitarians is useful if one is going to look into that literature. There are a significant number who think we should all go back to simpler lives [might be true] and view collapse as a good thing without thinking too much about it. There are some who view humanity as a curse and regard a population crash as a good thing, and don’t appreciate it if you point out that we are talking about billions of people starving to death live on TV. The main category is folk who realise that cheap energy is the core of modern civilization, and believe it is running out. To believe that you have to ignore nuclear power, and that is a recurring theme: endless discussion that refuses to discuss nuclear power at all [or underground coal gasification, which is starting to happen despite the CO2]. There are sensible people who think that our dependence on oil specifically will cause much more than an unpleasant change to different energy infrastructure as the oil runs out [e.g. Gail the Actuary]. There are those who think that our financial system will collapse because of the huge amount of debt that needs to be unwound by defaults or inflation [and it is certainly worrying that there aren’t decent economic models that incorporate things like the flow of sentiment]. Some expect climate change to negatively impact food production in a big way [makes no sense to me unless those unknown negative feedbacks overwhelm the positive feedbacks and make it cold and dry].

Understanding collapsitarians is useful if one is going to look into that literature.

I’m pretty familiar with the psychological and political dynamics of people who consider civilizational collapse or “going back to nature” an important or even desirable possibility.

Personally, I consider collapse scenarios important to ponder, and even fascinating in a certain grisly way, but not desirable.

I would like the population of the world to decrease gently, with a minimum of harm and suffering, ending in a state where everyone lives comfortably doing more or less what they want, with plenty of cities for people who like the intensity of urban bustle, plenty of villages for people who like those, plenty of farms for people who like to farm, plenty of beautiful parks for people to enjoy, and plenty of beautiful wild areas full of rich ecosystems. Not back to nature in the sense of reverting to primitive technology and scratching out a tough existence, but forwards to nature in the sense of developing technologies that give us what makes us truly happy with a minimum of waste and damage.

This is what I’d like, but it just possibly might not happen.

There are a significant number who think we should all go back to simpler lives [might be true] and view collapse as a good thing without thinking too much about it.

Yes, and I’m afraid Charles Greer, whose post on “catabolic collapse” I linked to, is a bit like that. He’s a pretty thoughtful guy. His more detailed paper:

is definitely worth reading. But his blog, while also fascinating, reveals a bit too much enthusiasm for collapse, to my taste. The fact that he’s an “Archdruid” may explain it.

(His blog is definitely worth reading, even if you disagree with everything he says and the concept of a modern-day “Archdruid” makes you cringe — because he can write well, and he’s talking about important things.)

The main category is folk who realise that cheap energy is the core of modern civilization, and believe it is running out. To believe that you have to ignore nuclear power…

Indeed Greer seems to make this mistake: he claims that when oil runs out, civilization as we know it will slowly collapse — but he seems to think that when you count the production of the power plants and waste disposal, nuclear power consumes more energy than it produces. I think the French would have noticed that by now, if it were true.

Some expect climate change to negatively impact food production in a big way [makes no sense to me unless those unknown negative feedbacks overwhelm the positive feedbacks and make it cold and dry].

It’s not just doomster druids who believe that climate change will negatively impact food production in some areas of the world. If you’re seriously interested in this issue, maybe it’s worth spending a few days reading this stuff:

The IPCC 2007 report has separate discussions on different areas of the world. For North America, the EPA provides a nice summary:

The IPCC concluded that, for North America as a whole (IPCC, 2007):

Moderate climate change will likely increase yields of North American rain fed agriculture, but with smaller increases and more spatial variability than in earlier estimates. Most studies project likely climate-related yield increases of 5-20 percent over the first decades of the century, with the overall positive effects of climate persisting through much or all of the 21st century.

* Food production is projected to benefit from a warmer climate, but there probably will be strong regional effects, with some areas in North America suffering significant loss of comparative advantage to other regions.
* The U.S. Great Plains/Canadian Prairies are expected to be particularly vulnerable.
* Crops that are currently near climate thresholds (e.g., wine grapes in California) are likely to suffer decreases in yields, quality, or both.
* Climate change is expected to improve growing conditions for some crops that are limited by length of growing season and temperature. (e.g. fruit production in the Great Lakes region and eastern Canada).

For Asia, where I live, the IPCC report says:

Results of recent studies suggest that substantial decreases in cereal production potential in Asia could be likely by the end of this century as a consequence of climate change. However, regional differences in the response of wheat, maize and rice yields to projected climate change could likely be significant (Parry et al., 1999; Rosenzweig et al., 2001). Results of crop yield projection using HadCM2 indicate that crop yields could likely increase up to 20% in East and South-East Asia while it could decrease up to 30% in Central and South Asia even if the direct positive physiological effects of CO2 are taken into account. As a consequence of the combined influence of fertilisation effect and the accompanying thermal stress and water scarcity (in some regions) under the projected climate change scenarios, rice production in Asia could decline by 3.8% by the end of the 21st century (Murdiyarso, 2000). In Bangladesh, production of rice and wheat might drop by 8% and 32%, respectively, by the year 2050 (Faisal and Parveen, 2004). For the warming projections under A1FI emission scenarios (see Table 10.5), decreases in crop yields by 2.5 to 10% in 2020s and 5 to 30% in 2050s have been projected in parts of Asia (Parry et al., 2004). Doubled CO2 climates could decrease rice yields, even in irrigated lowlands, in many prefectures in central and southern Japan by 0 to 40% (Nakagawa et al., 2003) through the occurrence of heat-induced floret sterility (Matsui and Omasa, 2002). The projected warming accompanied by a 30% increase in tropospheric ozone and 20% decline in humidity is expected to decrease the grain and fodder productions by 26% and 9%, respectively, in North Asia (Izrael, 2002).

Crop simulation modelling studies based on future climate change scenarios indicate that substantial loses are likely in rain-fed wheat in South and South-East Asia (Fischer et al., 2002). For example, a 0.5°C rise in winter temperature would reduce wheat yield by 0.45 tonnes per hectare in India (Lal et al., 1998; Kalra et al., 2003). More recent studies suggest a 2 to 5% decrease in yield potential of wheat and maize for a temperature rise of 0.5 to 1.5°C in India (Aggarwal, 2003). Studies also suggest that a 2°C increase in mean air temperature could decrease rain-fed rice yield by 5 to 12% in China (Lin et al., 2004). In South Asia, the drop in yields of non-irrigated wheat and rice will be significant for a temperature increase of beyond 2.5°C incurring a loss in farm-level net revenue of between 9% and 25% (Lal, 2007). The net cereal production in South Asian countries is projected to decline at least between 4 to 10% by the end of this century under the most conservative climate change scenario (Lal, 2007). The changes in cereal crop production potential indicate an increasing stress on resources induced by climate change in many developing countries of Asia.

If it gets colder and drier things will obviously be worse. We are encouraged to believe that if its warmer and (mostly) wetter it will also be worse. Obviously we are living at the peak of perfection. A scary place to be. No way to go but down.

If it gets colder and drier things will obviously be worse. We are encouraged to believe that if its warmer and (mostly) wetter it will also be worse. Obviously we are living at the peak of perfection.

You seem rather quick to dismiss a lot of work by a lot of qualified scientists.
Your argument neglects a couple of facts.

First: changes will cause problems even though we are not miraculously living at some peak of perfection, if those changes occur faster than we can easily adapt. Farmers have spent centuries adapting their methods of doing agriculture to the climate we have. So, any change in any direction can make things that work well now stop working so well. If the climate changes faster than farmers can adapt, life will become tough.

Second: you shouldn’t act as if the world is a unified place that will either get wetter or drier. All the predictions suggest that some parts will get wetter and other parts will get drier. For example, the eastern USA will see more floods, while the southwest USA will see more droughts. I believe this is happening already. If we could easily ship floodwater from the east coast to California, everyone would be happy. But we can’t.

So, it’s quite possible that changes will be negative on the whole, even if the world is not at ‘peak of perfection’.

But there are some big questions: Will changes be negative on the whole? Will changes occur faster than we can easily adapt to them?

There’s no way to settle these questions without doing a lot of work. Luckily, people have put a lot of work into them. If we’re serious about these issues, we should read what these people have written, and discuss it.

A scary place to be. No way to go but down.

I also find it odd that you’d make this sarcastic remark right after I took a lot of trouble to recommend the 2007 IPCC report, pointing out that it said this about the Americas:

I agree that the scientists should police themselves and no-nothing amateurs like me should shut up. For example, if some maverick says “the Arctic will be ice free in 5 years” it is important for the experts to make a press release saying “this is neither a consensus nor a majority opinion of climate scientists”. Otherwise when the prediction turns up false then the whole case loses traction. Obviously I don’t know what things to doubt, so I just get grumpy (and having shingles makes you grumpier).

It seems that humans can hopefully adjust by cultural means but the natural world will have trouble because so many are in smallish enclaves that are not interconnected. Also it is important to understand that the range of a species is misleading. Typically there is only a subset of the range, perhaps quite small, where the species has a positive reproduction rate (more than 2 children per female survive), and in the rest the rate is less than 2, but continually replenished by the overflow from the positive area. So it is easy to accidentally lose that positive area. And clearly climate change will be shifting where that positive area is.

[It is interesting that for humans, cities have always been population sinks, replenished from the country. This was always explained as being because cities were unhealthy. But now cities are very healthy, but it is still true. The reason is that cities are unsuitable for female reproduction. Women prefer to have a nice settled status relation with other women they meet before having children and that is impossible in the city.]

I assume that everyone realises that my final sentences in []s was just speculation. Not entirely irrelevant. The tendency of population to fall when nearly everyone lives in cities is the reason that we can hope that world population might stop rising. It would be wise to figure out why it works.

The global warming activist community seems to have set a CO2 target of 350 parts per million. For a web site dedicated for its popularization, see http://www.350.org.

For me, 350 ppm is a good enough working target. Now that we don’t have the technologies to reach the target is a big problem.

When Doctor Hansen asked the Japanese government to shutdown their coal powered electric plants, the reality was there was no replacement for coal, and he got an obvious answer of ‘no’. Nothing has changed since.

The German weekly newspaper “Der Spiegel” has published an interview with the physicist Hans Joachim Schellnhuber who is the head of the “scientific advisory board on global climate change of the German government”. Right now there is no online version available, and when it becomes available it will certainly be German only, here are some highlights:

1. Climate scientists should clearly point out the limitations of their knowledge and their models, lest they get dismissed as alarmist by the public, as has happened lately. Schellnhuber wishes that climate scientists could discuss their topics with the same freedom and passion that dominated the discussions of Bohr, Einstein et alt. about quantum mechanics, but says this is not possible because of the far reaching political implications and the scientific community, which is much bigger and more heterogeneous.

2. Climate change could have both negative and positive effects, but only the negative effects make it into the headlines, adding to the “alarmist impression”. Schellnhuber says that 90% of the funds go into the investigation if humans are responsible for climate change, which is a question that he considers to be settled (they are). Therefore, there should be much more effort to investigate the consequences of climate change.

3. His research committee advised that any temperature increase of more than 2 degrees Centigrade – compared to pre-industrial levels – should be avoided, for two reasons: First, politicians need clear-cut, quantitative answers, secondly, there is nothing known about a phase in earth’s history with higher temperatures, so that an increase above 2 degrees would take us into uncharted territory (the latter part is taken from a different interview with Schellnhuber that I remember, I don’t have any references for this claim, though).

4. It seems improbable that this 2 degree increase could be achieved without taking drastic measures like extracting from the atmosphere directly.

5. Last but not least he thinks that industrialized democratic societies need to take the lead in these issues, and, on a personal note, he suggests to give 10% of the votes in today’s parliaments to ombudsmen who have to represent the interests of future generations.

An addendum to point 2 above: Schellnhuber says that although there could also be positive effects, we should not take the chance because the risk that negative effects outbalance the positive effects is too high: Humans are performing a global climate experiment with unkown outcome, which is unwise.

He mentions that Vladimir Putin told him that Russians are actually glad about global warming, due to the special geographic conditions of their homeland. That was before the devastating bushfires aroung Moskau, which is an indication that we just don’t know enough about the consequences of climate change.

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